Brown spot (BS) breakouts represent the third most common disease of the Japanese rice crop; however, there are no new advances in breeding research for BS resistance. In this study, we developed a new method for evaluating field resistance to BS. In a test field, the susceptible paddy rice variety “Mienoyume”, which had previously been infected with BS, was transplanted uniformly (30 cm inter-row and 30 cm intra-row spacing) as the source of infection to generate a high infection pressure. On the basis of previous studies, the expanding lesion with a high ability to produce conidia was newly defined as ‘the lesion, with a grayish center part, or with wider width than that between the first-order veins of the rice leaf blade’. Multiple regression analysis, with the percentage of lesion area to leaf area as the target variable, and the number of all lesions and the percentage of the number of expanding lesions to that of all lesions as explanatory variables, demonstrated that the latter explanatory variable was more important than the former in explaining the target variable. The relationship between the percentage of lesion area and the percentage of the number of expanding lesions was analyzed, and a new ‘standard for evaluating BS severity’ was developed. This was divided into 10 index values (0–9) according to the percentage of lesion area and the types of lesions based on the percentage of the number of expanding lesions. Using this standard, the BS severity of recombinant inbred lines derived from crosses between the BS resistant variety “Tadukan” and susceptible variety “Hinohikari” was investigated four times after the booting stage. The frequency distribution of BS severity among the lines was the most similar to a normal distribution when investigated 113 days after transplanting. Using this method, which involved preparing a test field and evaluating BS severity, the results for BS severity in test materials were reproducible for several years. Accordingly, this method will be valuable for precisely evaluating BS severity in the field. Furthermore, using this method, some standard varieties for field resistance to BS were selected according to their maturation stages.